Doerr, one of the executives at firm Kleiner Perkins Caufield & Byers, remains on the board of Google, another company he backed in the 1990s.

Picking sides is becoming more common in a once-cooperative Silicon Valley. Google CEO Eric Schmidt quit Apple's board last August, as the two companies started to compete more in the smartphone business.

Now all three companies -- Google, Apple, and Amazon.com -- are competing with each other more and more.

Doerr cited time constraints, a frequent and anodyne reason given for board departures, and Amazon has said that it is "pleased with the current makeup of the board," though didn't rule out adding more directors.

Schmidt's earlier membership on both Apple and Google's board was cited as one of the possible reasons that the Federal Trade Commission has been investing anti-trust complaints about the companies. No one launched such complaints about Doerr's presence on Google and Amazon.com's board -- but had he stayed, it might only have been a matter of time.